Several types of nicotinic acetylcholine receptors (nAChRs) are known to play a role in central nervous system activity and as such are involved in cognition, mood and neuroprotection. The various types of known nicotinic ligands appear to have different combinations of effects on nicotine-modulated functions, depending on the subtypes of nAChRs affected, some affecting all receptors, others having more selective actions. A multitude of compounds has been investigated, including quinuclidines (AR17776 and congeners); azabicycyclic compounds for treating dementia (U.S. Pat. No. 5,217,975); 2-aroylaminothiazole derivatives that may be useful for treating cognitive disorders (U.S. Pat. No. 5,510,478); and 5-hydroxytryptophan receptor antagonists based on 1-azabicyclo nonane derivatives (U.S. Pat. No. 4,798,829). Published U.S. application (2004/0087616) discloses 1H-pyrazole and 1H-pyrrole-azabicyclic compounds reported to have alpha7 (α7) nicotinic acetylcholine receptor agonist activity which may be useful in treating the cognitive and attention deficit symptoms of Alzheimer's disease (AD) and other degenerative CNS conditions.
A large number of 3-arylidene-anabaseine compounds have been prepared (WO 2004/019943) for potential use in treating neurodegenerative diseases, and particularly with the hope that some compounds would bind to nicotinic alpha7 receptors. No particular nicotinic receptor activity (agonist or antagonist) or nicotinic receptor subtype selectivity has been demonstrated for any of these anabaseine analogs, all of which contain fused-ring heteroaromatic moieties attached through a methylene group to the 3-position of anabaseine without substitutions on the tetrahydropyridyl ring in the anabaseine molecule.
Acetylcholine receptors can be divided into muscarinic (mAChR) and nicotinic (nAChR) subtypes in the mammalian central nervous system (CNS). These subtypes are distinguished based on their ability to be stimulated by either the mushroom toxin muscarine or the plant alkaloid nicotine. Nicotinic receptors are important in cholinergic transmission in autonomic ganglia, striated muscles, the neuromuscular junction, and in brain and spinal synapses. Some nAChRs are also expressed in non-neuronal or muscle cells. Within the nervous system, these non-neuronal cells include microglia and astrocytes; outside the nervous system non-neuronal cells expressing alpha7 receptors include macrophages, vascular endothelium and pulmonary epithelial cells.
All known mammalian nAChRs are cation selective ligand-gated ion channels that form pentameric structures in the plasma membrane. Each subunit of the pentamer contains four transmembrane domains. There are at least seventeen different nAChR subunit genes, including five found in striated muscle (α1, β1, γ, δ, ε) and twelve neuronal nAChR subunits (α2-10, β2-4). These channels can be composed of a number of different combinations of subunits. Examples of the most abundant subtypes in the brain include the α7 subtype (α-bungarotoxin sensitive) and the α4β2 subtypes (α4(2) β2(3) or α4(3) ⊖2(2)). There is strong evidence supporting the idea that most α7 receptors are expressed as homopentamers. Functional bungarotoxin sensitive channels are expressed in Xenopus oocytes when only α7 cDNA is injected. However, rat hippocampal interneurons have α7-containing nAChRs that exhibit pharmacological and functional properties different from those of homomeric α7 receptors. The co-expression of the α7 subunit with the β2 subunit in Xenopus oocytes has produced functional heteromeric channels with similar properties to the rat hippocampal interneuron α7-containing receptor (Khiroug et al. 2004 J. Physiol. (London) 540:425-434). In addition to its ability to assemble into homomeric channels, the α7 nAChR channel displays much greater permeability to calcium ions than other nAChRs or the NMDA glutamate receptor subtype.
Neuronal nAChR deficits have been implicated in several diseases including AD and schizophrenia. Until recently, the study of neurodegenerative diseases focused on the muscarinic type neuronal acetylcholine receptor (mAChR) because of its abundance in the brain when compared to the population of neuronal nicotinic receptors (nAChRs). However, the discovery of a greater relative loss of nicotinic receptors than of muscarinic receptors in the Alzheimer's brain, as well as evidence that nicotinic agonists enhance cognition has spurred interest in nAChRs. This is supported by the observation of enhanced attentiveness and rapid information processing in humans receiving nicotine or DMXBA (GTS-21) treatment. The two major brain nAChRs alpha4beta2 (α4β2) and alpha 7 are important for cognitive processes such as attention, learning and memory. Since brain alpha7 nicotinic receptors are spared relative to the alpha4beta2 nAChRs in Alzheimer's disease and also possess exceptionally high calcium ion permeability, they are considered a particularly promising therapeutic target for treatment of Alzheimer's disease. In addition to their direct involvement in synaptic transmission, certain nicotinic receptor subtypes, particularly alpha7, because of their very high calcium permeability also stimulate calcium-dependent intracellular signal transduction processes that are neuroprotective by maintaining neuronal integrity in the presence of stressful states such as ischemia or mechanical trauma.
Central cholinergic neurons have been implicated in a number of neurodegenerative conditions including, AD and schizophrenia. AD affects an estimated 15 million people worldwide and accounts for approximately 50-60% of the overall cases of dementia for people over the age of 65. The characteristic pathology of AD includes extracellular β-amyloid plaques, intracellular neurofibrillary tangles, loss of neuronal synapes and pyramidal cells. The cholinergic dysfunction in AD is represented by a reduction in the activity of the ACh-synthesizing enzyme cholineactyltransferase (ChAT) and a loss in functional nAChRs. This alteration is possibly attributable to a reduction in nAChR synthesis, and/or to changes in nicotinic receptor pharmacology due to modifications in the binding site. In schizophrenia, there is a disruption in the normal brain mechanism that eliminates repetitive stimuli in order to reduce the flow of information. This malfunction in the simple filter for sensory input causes an overload of stimuli, which may lead to misperceptions of sensory stimuli producing delusions, or withdrawal from stimuli causing schizoid behavior.
It is now known that selective alpha7 nicotinic receptor agonists can improve memory-related behaviors and protect against neurotoxicity induced by trophic factor deprivation, amyloid exposure, excitotoxicity, in viva ischemia and axotomy (Li et al., 2000). The α7nAChR subtype is known to cause long-term synaptic modulation through its influence on glutamatergic synapses. Strong, brief stimulation of presynaptic α7-containing nAChRs can enhance hippocampal glutamatergic synaptic transmission for some time after the nicotinic agonist has been removed (Radcliffe and Dani, 1998).
DMXBA, 3-(2,4-dimethoxy benzylidene)-anabaseine is a well-studied compound that selectively activates alpha7 receptors in rats and has shown promise in Phase I human clinical trials. It also is an antagonist at alpha4beta2 receptors. DMXBA is less toxic than nicotine and does not affect autonomic and skeletal muscle systems at doses used to enhance cognitive behavior. Clinical tests of DMXBA indicate that large doses could be safely administered orally without adverse effects (Kitagawa et al., 2003. Neuropsychopharmacology 28:542-551; Olincy et al., 2006. Arch. Gen. Psychiat., in press).
Despite promising results in studies of anabaseine-related compounds such as DMXBA for potential treatment of cognitive disorders, these compounds penetrate into all tissues of the body, making them unsuitable for treating certain peripheral diseases. The action of DMXBA, for example, cannot be restricted to peripheral (accessible from the blood compartment) alpha7 receptors, which have recently been shown to have therapeutic importance for treating certain diseases.
The importance of developing highly selective alpha7 nicotinic receptor agonists has increased as the role of these receptors in degenerative disease becomes clearer. There is a particular need for new compounds useful in treating cognitive dysfunctions such as AD where degenerative processes drastically interfere with cognitive and physiological processes. Accordingly, compounds that are safe and are highly selective as alpha7 nicotinic receptor agonists would be prime candidates for therapeutics to treat human diseases involving neurodegeneration or defective development of the brain.
While some anabaseine-related compounds hold promise as alpha7 agonist drugs, they are not completely selective and can have antagonistic effects on brain alpha4beta2 subtype nicotinic receptors, which also participate in cognitive processes. Development of selective alpha7 agonists would allow less drug to be used, possibly with fewer side effects arising from interaction with other nicotinic receptor subtypes.
An additional advantage of new alpha7 agonist drugs would be identification of selective alpha7 agonists that do not penetrate into all tissues of the body, thus allowing their use in selectively targeting peripheral (accessible from the blood or pulmonary compartments) alpha7 receptors, which have recently been shown to have therapeutic importance for treating certain diseases.